1. Field of the Invention
The present invention relates to an aluminum alloy material with superior corrosion resistance, and to a plate heat exchanger using the aluminum alloy material in or as a heat transfer unit that uses a corrosive fluid, such as seawater, as a coolant (cooling medium).
2. Description of the Related Art
Aluminum (Al) alloys have high specific strength and high thermal conductivity and thus have been widely used as materials for compact lightweight heat exchangers. Representative examples of heat exchangers composed of aluminum alloys include fin-and-tube heat exchangers for use in household air conditioners and automotive radiators. In contrast, most of industrial single-pass heat exchangers using seawater as coolants are composed of titanium (Ti). Attempts have been made, however, to apply more economical aluminum alloys to such industrial single-pass heat exchangers.
Exemplary single-pass heat exchanges including heat transfer units using seawater as coolants (cooling water) include plate heat exchangers. They are exposed to stringent corrosive environments upon use in seawater environments. Thus, titanium (Ti), which has excellent corrosion resistance, is currently used. Aluminum alloys have high corrosion resistance as materials. However, when aluminum alloys are used for such plate heat exchangers in place of titanium, further sufficient corrosion protection is required, because they have not so high corrosion resistance as compared to titanium.
In general, examples of corrosion protection of aluminum alloys constituting such plate heat exchangers include formation of anodic oxide coatings, electrolytic protection, and formation of coatings with paint. Corrosion protection measures, if applied to heat exchangers, further include the incorporation of an inhibitor into a coolant.
However, plate heat exchangers are of a single-pass type, meaning that a coolant is discharged to outside of a system after passing through the apparatus and is not reused by circulation. Thus, a corrosion protection measure using an inhibitor is not proper, but a corrosion protection measure using paint film formation is economically appropriate.
Examples of coatings usable for aluminum alloys constituting heat exchangers include various types of inorganic, organic, and organic-inorganic hybrid coatings. These coatings are now practically used. Methods of forming coatings for heat exchangers are described typically in Japanese Unexamined Patent Application Publication (JP-A) No. 2003-88748 (Patent Document 1), JP-A No. 2004-42482 (Patent Document 2), JP-A No. 2006-169561 (Patent Document 3), and Akihiro YABUKI, Hiroyoshi YAMAGAMI, Takeshi OWAKI, Kiyomi ADACHI, and Koji NOISSHIKI, “Self-Repairing Property of Anticorrosive Coating for Aluminum Alloy”, Conference Proceedings of Material and Environment, 3-4 (2004) (Non-patent Document 1).
Patent Document 1 discloses the formation of a polyaniline coating for an aluminum alloy not constituting a plate heat exchanger using seawater as a coolant, to which the present invention is directed, but constituting a fin-and-tube heat exchanger for use typically in household air conditioners and automotive radiators.
Patent Document 2 discloses that, to improve adhesion, a coating is formed on a composite primer coating for an aluminum alloy material constituting a fin-and-tube heat exchanger for use in household air conditioners and automotive radiators as in Patent Document 1, which composite primer coating includes a coating prepared through treatment with a boehmite and/or a silicate.
Non-patent Document 1 discloses an anticorrosive trifluoroethylene polymer coating for a single-pass heat exchanger, which coating has self-repairing properties.
Patent Document 3 proposes a self-repairing aluminum alloy anticorrosive coating further containing 0.1 to 10 percent by volume of one or more members selected from zinc, titanium, manganese, aluminum, and niobium, in addition to such trifluoroethylene polymer. This technique is indicated as an improvement of the trifluoroethylene polymer anticorrosive coating. This is a measure for the fact that with a heat exchanger using seawater as a coolant, the surface of the heat exchanger is liable to be damaged, and, when surface damage is once induced, the damage tends to be abruptly enlarged by a vigorous corrosive action with seawater. More specifically, it is stated that the trifluoroethylene polymer anticorrosive coating containing a powder of the above-mentioned metal exhibits self-repair capability when the coating suffers damage.
The polyaniline coating disclosed in Patent Document 1 may be sufficient for an improvement in corrosion resistance of fin-and-tube heat exchangers used for household air conditioners and automotive radiators. However, when used in plate heat exchangers using seawater as coolants, to which the present invention is directed, such a coating is unsatisfactory with respect to seawater corrosion resistant properties such as corrosion resistance and coating adhesion in a saline environment such as of seawater.
The anticorrosive trifluoroethylene polymer coatings (fluorocarbon resin coatings) disclosed in Patent Document 3 and Non-Patent Document 1 have superior seawater corrosion resistance compared to the polyaniline coating disclosed in Patent Document 1 and to common corrosion protection such as anodic oxidation coatings and other coatings. However, when applied to a plate heat exchanger using seawater as a coolant, to which the present invention is directed, there arises a problem in that they degrade in adhesion (adhesion durability) to aluminium alloy materials in long-term use and are not thus reliable.
The degradation of adhesion (i.e., coating durability) to aluminium alloy materials in long use occurs likewise in primer or primer treatment that is directed to heat exchangers used in domestic air conditioners and automotive radiators such as of Patent Document 2. However, the fin-and-tube heat exchangers used in the air conditioners and automotive radiators have the life of at most ten and several years, and a required life of corrosion resistance is such a relatively short time as just mentioned.
In this connection, however, plate heat exchangers using seawater as coolants, such as vaporizers for natural liquefied gas, are industrially employed in plants, are of large-scale equipment and thus expensive. Accordingly, it is required that the life and anticorrosive life of the heat exchangers be a semipermanent life of several tens of years.
The corrosion resistance of such long life-oriented plate heat exchangers using seawater as coolants is predominant of adhesion of a coating to an aluminium alloy material rather than the corrosion resistance of the coating itself.
In this regard, the anticorrosive technique of providing a trifluoride resin anticorrosive coating (fluorocarbon resin coating) directly on a surface of aluminium alloy material as in Patent Document 3 and Non-patent Document 1 has a practical problem in that an adhesion to the aluminium alloy material is poor, and it is difficult to substantially improve the corrosion resistance under use of seawater.